The invention relates to an improved, lamination grade, coextruded, heat-sealable film. More precisely, the invention relates to a lamination grade, coextruded, heat-sealable multilayer film, having a core layer, a functional layer, and a heat-sealable layer, which exhibits excellent slip and machinability performance, as well as improved ink adhesion and lamination bond strengths, and which contains a non-migrating slip system requiring no aging time or temperature regulation.
Highly desirable properties of a single multilayer film include the ability to (i) function very well as both an outside and an inside web in a lamination (ii) exhibit excellent COF and machinability even after printing and laminating, (iii) exhibit excellent ink adhesion and bond strengths in lamination, and (iv) be prepared without migrating additives that require aging after production.
In general, a film used as an outside web in a lamination comprises a heat sealable layer with a relatively high melting point resin. The film used as an outside web tends to be less tacky or less sticky than the film used as an inside web. A film used as an outside web in a lamination should exhibit excellent machinability.
Improved film machinability allows greater amounts of film to be run through packaging equipment without jamming the equipment and also allows higher machine speeds to be used. Machinability can be tested by a “force over forming collar” test, wherein a film is pulled over a forming collar with force, and the higher the force needed, the less machinable the film is. Machinability can also be tested by a “hot slip” test, wherein the sealing bars of the packaging machine are heated up to 290° F., and the film is pulled over the forming collar with force. Again, the more force that is necessary, the less machinable the film is.
The most important property of a film used as an inside web in a lamination is the sealability of the film. The lower the minimum sealing temperature of the film, the broader the range of temperature that can be used to seal the film.
A film used as an inside web in a lamination should also exhibit good “hot tack.” “Hot Tack” is the strength of a heat seal immediately after sealing while still in a hot condition, i.e. before it has cooled down to ambient temperature and achieved its final strength.
Unmodified multilayer film with heat-sealable skin layers has an inherently high coefficient of friction (COF) and film-to-film blocking properties. Therefore, slip additives and antiblocking particulates are traditionally added to the film structure to lower the COF and provide improved machinability to produce, for example, food packages.
The slip properties of multilayer film have been beneficially modified by the inclusion of polymers of fatty acid amides, such as erucamide or oleamide. These fatty acid amide materials, however, disadvantageously depend on film temperature and storage time to promote the migration and effectiveness of this type of slip system. Fatty acid amide slip systems also have reduced functionality when the film is laminated to other non-slip containing films and the COF increases after lamination. Therefore, the production and functionality of fatty acid amide slip systems is limited.
Improved COF and slip functionality can also be gained by the incorporation of silicone oil into the skin layer of a multilayer film. Immediately upon winding a film with one skin layer containing silicone oil, the opposite side of the film structure is lubricated. Therefore, it is relatively easy to obtain a multilayer film with excellent slip performance on both sides when silicone oil is incorporated into the skin layer of a multilayer film. Films containing an appropriate concentration of silicone oil also tend to perform well in lamination on packaging machines and maintain a low COF.
A disadvantage with silicone oil slip systems, however, is the difficulty in converting a multilayer film that employs a silicone oil slip system. Silicone oil tends to transfer from one film surface to another upon winding of the film. Due to the silicone oil lubrication on both sides of the film, the treated surface becomes contaminated and consequently makes printing and ink adhesion more difficult. Additionally, if printing and laminating are done in two steps, i.e., out-of-line, then silicone oil can also transfer to the surface of the ink and cause future lamination bonding strengths to be low or inconsistent.